Method of manufacturing low density insulting refractories

ABSTRACT

Mineral wool, aluminum powder, hydrated lime, and plastic refractory clay are dry blended to form a homogenous mix, the homogenous mix blended with water and cast in molds, the molded mix vibrated and allowed to expand and set after which the molded article is dried and fired. In forming insulating refractories for higher temperature use requirements, pyrophyllite or alumina may be substituted for some of the refractory clay.

[52] U.S.Cl....,

United States Patent Booth [54] METHOD OF MANUFACTURING LOW DENSITYINSULTING REFRACTORIES [72] inventor: Alfred E. Booth, Lancaster, Pa,

[73] Assignee: Armstrong Cork Company, Lancaster, Pa.

22 Filed: May 27,1969 21 Appl.No.: 828,318

..l06/67, 106/40 [5 1] Int. Cl. l ..C04b 21/02 [58] Field of Search..106/40, 67; 264/43, 71

[56] References Cited UNITED STATES PATENTS 2,278,486 4/1942 Quigley etal 106/40 Mar. 14, 1972 Primary Examiner-James E. Poer Attorney-WilliamG. Taylor [57] ABSTRACT Claims, lprawing Figure PAIENTEDMAR 14 I972HYDRATED LIME PLASTIC REFRACTORY CLAY SIZING DUST MINERAL PYROPHYLL ITEFBER ALUMINA DRY MIXER WET BLENDER CAST IN MOLDS VIBRATE EXPAND AND SETREMOVE MOLDS POST- DRY CUT TO DESIRED SIZE KILN FIRING FINISHINGIINVENTOR ALFRED E- BOOTH ATTORNEY BACKGROUND OF THE INVENTION 1. Fieldof the Invention This invention relates to a method for manufacturinginsulating refractories, in particular insulating fire bricks.

2. Description of the Prior Art Insulating refractories haveconventionally been made from mixtures of refractory clays and sawdustand such mixtures have included pyrophyllite and alumina where use atvery high temperatures is required. The clay and sawdust are mixed withwater in a pug mill after which the mixture is conventionallymechanically molded in a wooden frame. The wooden frame is ordinarilyfilled to overflow and pressed. At this stage, the wood form or frame isremoved and the bricks are loaded onto pallets and delivered to a dryerwhere they are dried at about 300350 F. for 72 hours or more. Thesebricks are then loaded onto kiln cars which are moved continuouslythrough a conventional brick kiln. The brick kiln has three zones and inthe first zone, the sawdust is burned out to leave voids. In the secondzone, the bricks are fired at elevated temperatures to effect thedesired ceramic bonding and the third zone is the cooling zone in whichthe bricks are cooled at gradually decreasing temperatures. The overalltime required in the kiln is generally about 72 hours.

A slightly different system has been developed for forming insulatingfire brick wherein, in addition to the clay and sawdust mixconventionally used, an additional amount of plaster is added. In thisprocess, the dry mix including the plaster, clay and sawdust, and wheredesired pyrophyllite and alumina, is mixed with water and the wet mixpoured into molds. The mix readily sets or hardens in the mold and,after a short period of time, the mold can be removed by extruding theslabs directly onto a kiln car.

The loaded kiln car is charged directly into a high temperature dryer orpreferably into a combination dryer and kiln wherein drying temperaturesabove l,OOO F. may be utilized. By the time the end of the preheatingportion of the kiln is reached, all the sawdust has been burned out andthe bricks are then fired to the desired maximum operating temperature.Again the kiln has a zone to provide for gradual cooling of the bricks.

Obviously, the burnout of the sawdust in the abovedescribed methodsprovides a problem and requires considerable processing time. In thesecond method described, the decomposition of the gypsum plasterprovides added difficulties in processing, although some time saving isrealized through the use of higher drying temperatures.

SUMMARY OF THE INVENTION I have discovered that insulating refractoriesof required low densities may be formed without the necessity ofincorporating burnout material, such as sawdust, therein. In accordancewith the invention, mineral wool, aluminum powder, hydrated lime,particles of plastic refractory clay, and alumina and/or pyrophyllitewhen desired, are dry blended, mixed with water to form a moldable mixwhich is poured or cast into molds and vibrated. The molded mix isallowed to expand and set after which it is dried to a moisture contentof about to percent. At this point, the slab stock is readily removedfrom the molds. The slab stock is put on conventional conveyingequipment and transferred to a kiln wherein it is fired at a temperaturewithin the range of from about 2,200 to 2,850 F. for anywhere from about2 to 6 hours dependent on end use temperature requirements and mixcompositions.

DESCRIPTION OF THE DRAWING The FIGURE of the drawing is a flow diagramsetting forth schematically the process ofthis invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The description of thepreferred embodiments is made with reference to the flow diagram.

Based on the total weight of dry ingredients, the basic mix containsfrom about I to 7.5 percent by weight inorganic fiber, about 0.2 to 1.0percent by weight aluminum powder, about 1 to 10 percent by weighthydrated lime, and about 30 to 97 percent plastic refractory clay. Up toa maximum of 25 percent by weight of pyrophyllite may be substituted forpart of the clay in the manufacture of 2,600 F. rated fire brick and upto a maximum of about percent by weight of alumina may be substitutedfor part of the clay in the manufacture of 2,800 F. rated fire brick.

Scrap from any of the steps, through drying the slab stock. may be usedin the basic mix and the amount of scrap utilized is not critical sincethe ingredients on mixing and wet blending will be in essentially theform of the original dry ingredients although minor adjustments in theamount of aluminum powder and hydrated lime will have to be made wheremuch scrap from this source is used in the mix.

In addition, fired sizing dust from the finishing operation may beutilized in the dry blended mix up to a maximum of percent by weight ofthe dry blend mix.

The mixture is initially dry blended to form a homogenous mix which isadded directly to a wet blender, such as a J. Hv Day Co. Centri-FloMixer, at which point water is added to give a wet blend of anessentially creamy consistency. The amount of water to be added willvary with the plasticity of the blend, but generally the addition ofabout 1 part by weight of water to 1 part by weight ofdry blend isrequired. I have found that wet blends having an apparent viscosity inthe range of about 12 to 23 poises, at a shear rate of sec., form mixesof the desired consistency. The mix formed at the wet blender istransferred directly to casting molds, bottomless molds carried on apallet such as cement-asbestos board, and the molds are partiallyfilled, usually to a maximum of about 50 percent of the volume of themold. At this point, the molds are preferably vibrated to remove theentrapped air and the vibration also apparently aids the expansion andsetting reaction of the mix. The wet blended mix in the molds goesthrough an exothermic reaction and the mix expands to essentiallycompletely fill the mold and achieve an initial set.

The casting molds having the set mixed formed in slabs therein are thentransferred to a dryer and control dried to approximately a 20 to 25percent moisture content, usually at a temperature of about 500 F.Alternatively, the cast molds may be predried to remove some of thewater and to further set the mix. At this point, the slabs have shrunkand the mold forms can be removed from the cast slabs. The slabs remainon the pallet and are then passed through a postdryer which removes theremaining water necessary to give a final moisture content of from 20 to25 percent moisture by weight. In either instance, after drying, theslabs are in a condition such that they may be readily cut with saws orwires to any desired size.

The dried slabs, cut as desired, are next loaded onto a suitableconveyor, such as a conventional brick kiln car, and transported to akiln where they are fired to the desired temperature, usually atemperature within the range of from about 2,200 F. to about 2,850 F. Onremoval from the kiln, they may be finished by any conventional brickfinishing operation such as grinding or cutting with bandsaws.

In the practice of the invention, we prefer to utilize a commercialgrade mineral wool although other inorganic fibers such as relativelyfine glass fibers, asbestos fibers and refractory fibers such asFiberfrax, a high alumina fiber produced by Carborundum Company, may beused equally as well. The use of fine fibers is preferred because thefunction of fibers is primarily to make the expanded product moredimensionally stable on first set.

As the plastic refractory clay, kaolin is preferred although other clayssuch as Pine Lake clay and various Fire clays may be utilized,especially for lower temperature uses. The clay used must have a degreeof plasticity such that it entraps and retains the gas formed from thegas producing reaction after the ingredients have been wet blended. Theclay must also possess the refractory properties necessary at the usetemperature such that specified reheat shrinkage requirements are met bythe fired product when tested by ASTM C 210.

The aluminum powder preferred is a very fine particle size, nonleafingaluminum powder sold by Alcan Metal Powders, Inc. (Grade 3,100).

As stated above, enough water is added at the wet blending step to forma mix of creamy consistency. The amount of water required will vary withthe amount and plasticity of the clay in the mix. Generally, based onparts by weight of dry blend, the amount of water required will be inthe range of from about 7 to parts by weight. The higher amounts ofwater are required for mixes utilizing relatively large amounts ofthemore plastic clays.

In order to speed the reaction but yet control it to the desired extent,the water added is preferably at a temperature of about 120 F.Obviously, cooler water may be used but this increases the dwell timenecessary to expand and set the mix after it has been poured into themolds. With water temperatures greater than 140 F. the reaction isspeeded up to the point where it is difficult to control and the cellsformed may become too large.

A measured amount of wet mix is put into the mold so that the slabformed on expansion fills the mold cavity. Usually the wet mix requiredfor forming a slab which fills the mold occupies about to 50 percent ofthe volume of the mold, the amount of expansion depending on mixcomposition. The mold, on filling, is vibrated, preferably with a highfrequency low amplitude vibration, which acts to remove entrapped airfrom the mix and also has been found to aid the reaction and expansionof the mix in the mold. Usually, expansion is complete within 2 to 5minutes of filling the molds and the mix sets or hardens to a desireddegree of hardness in about 20 minutes. The degree to which the mix setsis not a critical limitation but does affect the ease of handleabilityof the filled molds.

Preferably the drying temperature utilized is about 500 F. although theuse of higher temperatures (up to a maximum of about 800 F.) appearsfeasible. During drying, the slab stock in the molds is dried to amoisture content of about 20 to 25 percent although this range could beextended to from about 0 percent moisture to not more than percent byweight moisture. The lower the moisture content, the more friable theslab. Generally at 500 F. a drying time of about 1% hours is required.although this will vary slightly depending upon the particular mix usedfor forming the slab stock and the final moisture content. In apreferred embodiment, the molded slab stock is predried only to theextent necessary to shrink the slab stock away from the mold. At thispoint, the molds are easily removed from the slab stock. The slab stockis then put on a pallet and cycled through a postdrying cycle where theremainder of moisture is removed to give a slab stock of preferably 20to 25 percent moisture content. This alternative method results in anadvantage in that iii-process mold usage is minimized.

After drying, the slab may be cut to any desired size and all the scrapat this point may be recycled to the basic mixing operation and may beused in any proportions in the basic mix. Sizing dust from the finalfinishing operation and after the firing cycle is also used in theoriginal dry mixing but in limited quantity. The sizing dust afterfiring is refractory and only up to 50 percent by weight ofthe dry blendmix may be utilized.

The dried slab stock is next loaded onto a suitable conveyor such as aconventional brick kiln car and transported to the firing kiln. Aconventional brick firing kiln is divided into a heat up zone, a firingzone and a cooling zone. For brick rated at 2,000 F. or 2,300 F. theslab stock is fired within a temperature range of 2,200 to 2,400 E,usually at a minimum of about 2 hours for the 2,000" rated brick andabout 4 hours for the 2,300 F. rated brick. For 2,600 F. rated brick, aminimum firing period of about 4 hours at a temperature within the rangeof 2,450 to 2,650 F. is required and for the 2,800 F. rated brick, aminimum firing time of about 6 hours at a temperature within the rangeof 2,650 to 2,850 F. is required.

The heat up time required is usually about onehalf hour and a coolingperiod ofabout 2 hours is usually required to insure against thermalshock.

The following table describes dry mix formulation for lightweight firebricks rated for varying use temperatures.

TABLE 1 Parts by Weight For each of bricks (A), (B) and (C), the givenmaterials were dry-blended to form a homogenous mix and the homogenousdry mix was then thoroughly blended with about F. water to form a wetblend of creamy consistency which was immediately cast into molds,vibrated and allowed to expand and set. To form a blend of the desiredconsistency. a ratio of approximately l.5 part by weight water to l partby weight dry mix was used for (A), a ratio of approximately 1 part byweight water to 1 part by weight dry mix was used for (B) and a ratio ofapproximately 4 parts by weight water to 5 parts by weight dry mix wasused for (C). The wet blends had initial apparent viscosities of.respectively, (A) about 20 poises at a shear rate of 55 sec.', and (B)and (C) about 15 poises at a shear rate of 55 see". The amount of wetblend added to the molds varied from about 25 percent by volume for (A)to about 50 percent by volume for (C) and the time required forexpansion and achieving initial set varied from a maximum of about 20minutes for (A) to a minimum of about 5 minutes for (C).

Drying of the respective slab stock thus formed was carried out in adryer at 500 F. for about 1 /2 hours at which time, on removal from thedryer, the molded slab stock had a moisture content of about 25 percentby weight.

The slabs were removed from the mold and loaded onto a conventionalbrick kiln car and the slabs cycled through a conventional brick kiln.

The time and firing temperatures required in the kiln vary depending onthe mix used and the use temperature requirements of the product. Foreach. the heat up time prior to firing is about one-half hour and thecooling down period required is about 2 hours. The gradual heating up ofthe dried slabs in the kiln and the gradual cooling down of the firedproduct is necessary in avoiding the possibility of thermal shockoccurring in the slab stock during processing.

Mix (A) set forth in Table l is designed for fire brick rated at 2,000F. and 2,300 F. The firing time at temperature required for 2,000 F.brick was about 2 hours at about 2,300" F. and the firing time requiredfor 2300 F. brick was about 4 hours at about 2,300 F. For the 2,600 F.brick [Mix (8)] a firing time of about 4 hours at about 2,550 F. wasrequired and for the 2,800 F. brick [Mix (C)] a firing time of about 6hours at about 2,750 F. was required.

All of the bricks produced in accordance with the above were of lowdensity, good strength and uniform pore structure as compared with aragged pore structure for fire brick produced using a burnout technique.Physical properties for the various brick are set forth in Table IIbelow.

TABLE ll A\erage (A) (B) (C) 2000 F. and 2300 F. 2600 F. 2800 F.

Brick equivalent weight 1.5 to 1.65 2.6 2.8 Reheat shrinkage 0.1192 lessthan less than (24 hr. at I950F.) 0.5%(a) l.0%(b) Crushing strength I40psi more than more than 200 psi 200 psi Brick equivalent weight is theweight in pounds ofa brick measuring 21z 4% 9.

a. 24 hours at 2550 F.

.24 hours at 2750 F.

B. blending said mixture with sufficient water to provide a wet blendhaving an apparent viscosity in the range of about 12 to 23 poises at ashear rate of55 secf";

C. partially filling a mold with said wet blend;

D. vibrating said molds to remove entrapped air and allowing thevibrated mix to expand and set into a slab;

E. drying said slab to a maximum of about 35 percent by weight moisture;and

F. firing at a temperature within the range of about 2.200

to 2,850 F.

2. A method in accordance with claim I wherein up to 50 percent byweight of a fired sizing dust of the composition of the refractoryproduced in accordance with claim 1 is dryblended with the ingredientsrecited in claim 1.

3. A method in accordance with claim 2 wherein the slab is dried to amoisture content of 20 to 25 percent by weight.

4. A method in accordance with claim 2 wherein said inorganic fiber ismineral wool and wherein said aluminum powder is of extremely fine,nonleafing particle size.

5. A method in accordance with claim 4 wherein the water to dry mixratio is about 7 to 15 parts water to 10 parts dry mix and wherein thewater temperature at addition is between about F. and F.

6. A method in accordance with claim 5 wherein kaolin clay is utilizedand wherein the slab is fired at a temperature in the range of about2,200" to 2,400" F. for a minimum firing time of about 2 hours.

7. A method in accordance with claim 5 wherein a mixture of kaolin clayand pyrophyllite is utilized and wherein the slab is fired at atemperature in the range of about 2,450 to 2,650 F. for a minimum firingtime ofabout 4 hours.

8. A method in accordance with claim 5 wherein a mixture of kaolin clayand alumina is utilized and wherein the slab is fired at a temperaturein the range of about 2,650 to 2,850 F. for a minimum firing time ofabout 6 hours.

2. A method in accordance with claim 1 wherein up to 50 percent byweight of a fired sizing dust of the composition of the refractoryproduced in accordance with claim 1 is dryblended with the ingredientsrecited in claim
 1. 3. A method in accordance with claim 2 wherein theslab is dried to a moisture content of 20 to 25 percent by weight.
 4. Amethod in accordance with claim 2 wherein said inorganic fiber ismineral wool and wherein said aluminum powder is of extremely fine,nonleafing particle size.
 5. A method in accordance with claim 4 whereinthe water to dry mix ratio is about 7 to 15 parts water to 10 parts drymix and wherein the water temperature at addition is between about 100*F. and 140* F.
 6. A method in accordance with claim 5 wherein kaolinclay is utilized and wherein the slab is fired at a temperature in therange of about 2,200* to 2,400* F. for a minimum firing time of about 2hours.
 7. A method in accordance with claim 5 wherein a mixture ofkaolin clay and pyrophyllite is utilized and wherein the slab is firedat a temperature in the range of about 2,450* to 2,650* F. for a minimumfiring time of about 4 hours.
 8. A method in accordance with claim 5wherein a mixture of kaolin clay and alumina is utilized and wherein theslab is fired at a temperature in the range of about 2,650* to 2,850* F.for a minimum firing time of about 6 hours.